// JS的语法
const a = (param = 2) => 1 + param
const b = [ 1, 2, 3 ]
const c = [ ...b, 4, 5 ]

class Babel {}

new Babel()

// JS的API
const x = [ 1, 2, 3, 4, 5 ].filter(x => x % 2 === 0)
const y = Object.assign({}, { name: 1 })
// 目标：把箭头函数、...、class类转成es5
